The invention relates generally to the field of cycles, and in particular to bicycles. More specifically, the invention relates to an electric assist bicycle which is configured to maximize the efficiency of the motor and to prolong the life of the battery which supplies electrical current to the motor.
Over the last 150 years, the bicycle has evolved to become one of the most efficient means of transportation in terms of conversion of energy into distance traveled. For example, most modem bicycles require only about 400 watts (xc2xd horsepower) to propel the bicycle at 15 m.p.h. on level ground. The efficiency of the bicycle has also been optimized to minimize the effort required by the rider. For instance, most modem bicycles include an efficient gear system to minimize rider effort.
To further reduce the amount of human effort required to propel a bicycle, a variety of electric bicycles have been introduced. Presently, about 50 to 100 companies are producing or are planning to produce electric bicycles. In most cases, however, such bicycles do not utilize the efficiency of the bicycle through the use of mechanical gears.
The human muscle and modem battery are similar in their ability to produce power from stored energy. Similarly, both are able to produce more energy by keeping the torque per stroke low and the frequency high.
The human muscle is able to function in two states: anaerobic or aerobic. In anaerobic contraction, the muscle utilizes stored ATP fuel to power the muscle without the need for oxygen. In this case, the muscle can produce large amounts of energy for a short duration. The byproduct of this high energy output is lactic acid. As muscle contraction continues in an anaerobic state, the lactic acid in the muscle builds until it inhibits further muscle contraction. After a period of rest, the lactic acid is removed from the muscle by the blood system and muscle contraction can continue (assuming a sufficient store of ATP fuel). Aerobic muscle contraction allows for extended periods of exertion, but at a lower level of power than anaerobic exercise. In aerobic exercise, sufficient oxygen is supplied to the muscle so that the muscle is able to use the soluble fat in the blood as the primary fuel.
The gears of modern bicycle allow the rider to exercise the muscle in the aerobic range to allow continuous long distance riding. The gears are utilized to keep the rider""s pedal speed at a high rotating speed (usually between about 60 to 100 rpm). At higher pedaling speeds, the force output for muscle contraction is low so that the muscle is able to stay in the aerobic region.
The original bicycle used a single fixed gear ratio (similar to most electric bicycles) and was severely limited in its ability to negotiate steep terrain. The number of gears on a bicycle has evolved so that the present mountain bike has up to 27 gears to allow for riding on a variety of terrains.
Similar to the human muscle, the modem battery has an efficient and an inefficient region. The battery delivers current to the motor, which produces torque in the motor. The motor torque increases linearly with motor current. High currents are inefficient.
At high current discharge rates, the battery experiences problems similar to lactic acid buildup in the human muscle. More specifically, in the battery, hydrogen gas is formed on the charge plate. Hydrogen gas acts as a barrier to the transfer of electrons. As the high current discharge continues, the hydrogen continues to build on the plates until the battery is unable to deliver current.
Another important issue to consider at high current discharge rate is that the run time of the battery is reduced exponentially with linear increases in motor current. Further, motor thermal losses are experienced which increase with the square of the motor current. Hence, increased motor current wastes available energy two non-linear ways, i.e., battery losses and motor resistance losses.
As one example, a motor mounted directly to the rear wheel on the bicycle has only a fixed gear ratio. Hence, to obtain a four times increase in torque, the motor current must be increased by four times. However, the four times increasein the motor current increases motor resistive losses by 16 times and thus results in a significant loss in battery run time and reduction in motor efficiency.
The available power from the battery is an exponential function of the rate of current use. Hence, as current discharge increases, the available energy from the battery decreases exponentially. Hence, as more torque is required to move the bicycle (such as during hill climbing or acceleration), more current will be required, thereby exponentially decreasing the available power from the battery.
Hence, it would be desirable to provide improved electrically assisted bicycles and methods for their use which would overcome or greatly reduce these and other problems. The electric bicycles of the invention should be configured to maximize the efficiency of the motor, minimize current use, and thus maximize battery life. It would be desirable if such features could be accomplished by minimizing the required torque while keeping the rotational rate of the motor as high as possible. Preferably, the electric bicycles of the invention will employ the use of a gear system so that torque may be minimized, especially during hill climbing and acceleration. It would further be desirable if the electric bicycles of the invention provided for automatic shifting to keep the motor speed near maximum output while minimizing torque. In another aspect, it would be desirable if such electric bicycles were able to operate using either the motor or the pedals in a parallel manner. At the same time, it would be preferable if such electric bicycles employed the use of a motor which did not turn the crank arms. Such electric bicycles and methods should also be compatible with conventional bicycle equipment, such as derailleurs so that shifting may be accomplished with minimal modification to existing bicycles. Finally, it would be preferable to incorporate the batteries into the bicycle in a manner such that the overall appearance of the bicycle is aesthetically pleasing, such the batteries are protected, and such that the bicycle is provided with a low center of gravity.
The invention provides exemplary electric motor assemblies, electrically assisted bicycles, and methods for their use. In one exemplary embodiment, the invention provides an electric motor assembly which comprises a housing and a spindle that is disposed to rotate in the housing. A motor is disposed within the housing and comprises a stator coupled to the housing and a rotor rotatably disposed within the stator such that the rotor is disposed about the spindle. The motor assembly further includes an output driver, and a gear system operably coupled to the rotor and the output driver to rotate the output driver upon operation of the motor.
The disposition of the motor and output driver within the housing is advantageous in that it facilitates packaging and manufacturing of the motor assembly. Preferably, the spindle is aligned with a central axis of the housing, with the rotor being concentrically disposed about the spindle, and the stator being concentrically disposed about the rotor. Such a configuration allows for a compact design to allow the motor to conveniently fit within the housing.
In another particularly preferable aspect, a front sprocket assembly is operably coupled to the output driver such that the sprocket assembly rotates upon rotation of the output driver. By having the motor turn the sprocket assembly, the motor assembly may be used in connection with mechanical gears of the modern bicycle to minimize the amount of torque required, thereby greatly increasing battery life.
In another particular aspect, the gear system is coupled to a motor driver. The motor assembly further includes a first clutch to engage the motor driver with the output driver when the motor driver is rotated faster than the output driver. In this way, when the rider is pedaling at a rate which causes the output driver to rotate faster than the motor is turning the motor driver, the first clutch will not engage the motor driver with the output driver. Hence, the rider is able to pedal the bicycle and not turn the motor. Conversely, if the motor turns the motor driver at a rate which is faster than the rider is pedalling, the first clutch is engaged so that the motor causes the output driver (and hence the sprockets) to rotate. Optionally, another clutch mechanism may be provided which allows the rider to engage the clutch during pedaling for regenerative charging of the battery.
In yet another aspect, a crank arm is coupled to the spindle, and a pedal is coupled to a crank arm. A second clutch is also provided to engage the crank arm with the output driver when the crank arm is rotated faster than the output driver (thereby releasing the first clutch) so that the rider""s legs cause rotation of the output driver. Use of the second clutch is also advantageous because, when the motor is turning the output driver, the second clutch will ensure that the crank arm is disengaged. In this way, the motor is able to turn the sprocket assembly but not the crank arms. Preferably, the first clutch and the second clutch are coaxially aligned with an axis of the spindle to allow for packaging of the motor in the small space available between the crank arms.
In yet another aspect, the gear system comprises a set of planetary gears to rotate the output driver at a rate of rotation that is less than the motor. Preferably, the gears are configured so that the output speed of the motor is matched to the range of the human leg. For example, the planetary gears are preferably configured so that when the rate of rotation of the motor is in the rate from about 1,800 rpm to about 3,600 rpm, the rate of rotation of the output driver is in the range from about 60 rpm to about 120 rpm. In a specific aspect, the motor speed is approximately 2400 rpm and is employed to turn the crank arms at a rate of about 75 rpm. Such a gear reduction facilitates use of either the motor or pedal power to drive the bicycle. The motor is preferably operated at or near its maximum output level to maximize the efficiency of the motor and minimize current use, thereby prolonging the life of the battery. Operating the motor at or near its maximum output level is also advantageous in that the motor is able to generate more power at higher rates of rotation.
In still yet another aspect, the motor comprises a brushless DC motor. Such a motor is preferable because it provides superior cooling and a high power output. Alternatively, a brushed or SR motor may be used.
In one particular aspect, at least one bearing assembly is coupled to the housing and disposed about the spindle. In this way, the pedals are free to turn when operated by a rider. Use of the bearing assembly is also advantageous in that the crank spindle is used to support the rotor and the planetary gears. Another bearing assembly is preferably disposed between the rotor and the spindle so that rotation of the rotor is generally prevented upon rotation of the spindle by the crank arm. In this way, the rider may pedal the bicycle without turning the motor. Also, this bearing assembly prevents the spindle, and therefore the crank arms, from rotating when the motor is operating.
The invention further provides an exemplary cycle which comprises a frame having a bottom bracket. At least one wheel is operably coupled to the frame. The bicycle further includes a motor assembly that is disposed within the bottom bracket. Preferably, the motor assembly is constructed to be similar to the motor assembly just described. A first sprocket assembly is coupled to the output driver of the motor assembly such that the sprocket assembly rotates upon rotation of the output driver. A second sprocket assembly is coupled to the wheel, and a chain is coupled between the first sprocket assembly and the second sprocket assembly to rotate the wheel upon rotation of the output driver.
The disposition of the motor assembly in the bottom bracket is particularly advantageous in that the motor is housed at a low center of mass of the cycle. Advantageously, the motor is not disposed on the wheel which may otherwise add unsprung mass and cause poor suspension and handling and added rotational dynamics. By packaging the motor in the bottoming bracket, the motor is extremely efficient.
In one particularly preferable aspect, the frame defines a cavity, and at least one battery is housed within the cavity and is electrically coupled to the motor. Preferably, the bicycle frame is constructed of a monocoque design having a hollow center for receiving the battery. In this way, the battery may be mounted in front of the bottom bracket motor and low on the bicycle frame so that the center of mass of the bicycle is low. Further, such a configuration allows the battery to be loaded from the bottom of the bicycle and allows for easy removal. Further, the battery pack and its supports becomes an integral part of the structural strength of the frame when secured within the frame.
In another aspect, the second sprocket assembly includes multiple gears, and a shifting mechanism is provided to move the chain between the gears. In this way, the bicycle may be shifted between gears to minimize the required torque. In turn, less current is required so that the life of the battery may be prolonged. Conveniently, a controller may be provided to control actuation of the shifting mechanism based on the rotational wheel speed and the rotational speed of the first sprocket assembly. In this way, the motor may be kept at maximum speed by shifting the gears. In this manner, the efficiency of the motor is maximized.
Advantageously, due to the first clutch in the motor, the chain may be shifted between the gears of the second sprocket assembly while the cycle is coasting. This is because the motor is able to turn the front sprocket assembly while the cycle is coasting (and without turning the pedals). Such a feature is advantageous in that the cycle is able to be placed in the appropriate gear which corresponds to the current wheel speed. Further, by the time the rider comes to a stop, the controller has placed the chain in the lowest gear so that starting torque and acceleration may be increased. Similarly, when climbing hills, the controller may be employed to shift down so that more torque may be provided to the rear wheel without using excessive current.
Conveniently, the shifting mechanism may comprise a derailleur and a cable that is coupled to the derailleur. A stepper motor is provided and has a lead screw to tension the cable based on signals received from the controller. In this way, the cycle may include a standard derailleur which in turn is employed to shift the gears when the cable is moved by the stepper motor upon receipt of signals from the controller.
In yet another aspect, the cycle includes a throttle to control the speed of the motor. Conveniently, the throttle may comprise a potentiometer that is mounted within a handlebar. The use of an internal potentiometer is particularly advantageous in that it does not interfere with conventional bicycle shift mechanisms which may optionally be employed to shift the chain between the gears.
In one particular aspect, a swing arm is pivotally coupled to the frame, and the wheel is attached to the swing arm. A suspension mechanism is also disposed between the swing arm and the frame. Such a configuration is made possible by including the motor in the bottom bracket so that it does not interfere with the rear suspension.
The invention further provides an exemplary method for operating a cycle. According to the method, the cycle has a frame and at least one wheel coupled to the frame. A front sprocket assembly is rotatably coupled to the frame and a rear sprocket assembly is coupled to the wheel. A chain is positioned between the first sprocket assembly and the second sprocket assembly. A motor assembly is provided and has a motor driver to turn the first sprocket assembly and a crank arm to turn the first sprocket assembly. Such a cycle is operated by actuating the motor and optionally turning the crank arm. The motor is engaged to turn the first sprocket assembly if the motor driver is turning faster than the first sprocket assembly. However, if the crank arm is rotated faster than the first sprocket assembly, the crank arm is engaged with the first sprocket assembly. In this way, the rider may choose to have the motor drive the bicycle simply by not turning the crank arm. When the rider wishes to operate the bicycle using human leg power, the rider simply turns the crank arm until the first sprocket assembly is rotating faster than the motor driver. Preferably, when the rider begins to turn the crank arm, such action will not cause the motor to rotate.
In one particular aspect of the method, a second sprocket assembly includes multiple gears. In this way, the gears are shifted to maintain the motor speed at a near maximum output level while the front sprocket assembly rotates at a rate within the range of the human leg. In this way, the user is able to take over propulsion of the cycle by simply pedaling faster than the motor driver as previously described. Preferably, the motor is operated at a rate in the range from about 1,800 rpm to about 3,600 rpm, and the front sprocket assembly is turned at a rate in the range from about 60 rpm to about 120 rpm.
In one particularly preferable aspect, the gears are shifted without turning the crank arm. This is made possible by having the motor turn the front sprocket assembly without turning the crank arm.